K. S. Khalina, “Boundary controllability problems for the equation of oscillation of an inhomogeneous string on a semiaxis”, Ukr. Mat. Zhurn., vol. 64, no. 4, pp. 525–541, Apr. 2012, Accessed: Feb. 16, 2026. [Online]. Available: https://umj.imath.kiev.ua/index.php/umj/article/view/2593